Printing plates used in flexographic printing typically comprise a photopolymer layer or some other photosensitive imageable layer. Traditionally, an image is applied to the printing plate by exposing the photopolymer layer to actinic radiation (e.g. ultraviolet (UV) flood radiation) with a UV-opaque image mask interposed between the radiation source and the printing plate. Because of the image mask, some regions of the photopolymer are exposed to the radiation while other regions of the photopolymer are not exposed. The exposure to actinic radiation changes the properties of the photopolymer in the exposed region, for example by cross-linking. After imaging, the plates are processed to remove either the exposed photopolymer regions or the unexposed photopolymer regions, thereby creating a relief-based image on the printing plate. The processed plates are then mounted on a printing press, where they are used to transfer ink to desired printing surface(s).
Recently, it has become possible to prepare flexographic printing plates with an integral UV-opaque surface mask layer. Such UV-opaque surface mask layers are typically sprayed or roll-coated over the photopolymer surface after the plate has been fabricated and then imagewise patterned to form a mask. Imagewise patterning may involve ablating or removing masking material from the plate or otherwise converting masking material between UV-transparent and UV-opaque states.
One type of integral UV-opaque surface mask layer is known as laser ablation mask system (“LAMS”) layer. In operation, the UV-opaque LAMS layer is coated onto the top surface of the printing plate's photopolymer layer and is then selectively (i.e. imagewise) patterned. Patterning the LAMS layer typically involves selective laser ablation of the LAMS layer. Such laser radiation may be provided by infrared (IR) lasers on a laser imaging head which is part of a digital imaging device, for example. Regions of the LAMS layer which have been ablated in this manner become UV-transparent, creating a mask that is integral with the photopolymer layer of the printing plate. After ablating the LAMS layer to create the mask, the plate is exposed to actinic flood radiation. In regions where the LAMS layer has been ablated to become UV-transparent, the underlying photopolymer layer is exposed to the actinic radiation. Conversely, in regions where the LAMS layer has not been ablated and is still UV-opaque, the underlying photopolymer layer is not exposed. The exposure to actinic radiation changes the properties of the photopolymer in the exposed regions. The plates are then processed to create a relief-based image on the printing plate by removing the LAMS layer and either the exposed or unexposed photopolymer regions. The processed plates may then be used in a printing press to transfer ink to desired printing surface(s).
Further advances in the art have made it possible to provide many plate preparation procedures “in the round” (i.e. on the cylindrical surface of a drum rather than on a flat two-dimensional surface). Such “in the round” procedures may involve, for example, applying a surface mask layer to the photopolymer layer of a printing plate, imagewise ablating or otherwise imagewise patterning the surface mask layer, exposing the masked plate to actinic flood radiation and subsequent processing.
In order to perform these types of plate preparation procedures “in the round”, it is desirable for the printing plates to be easily supported on the cylindrical surface of a drum. One technique, known as continuous photopolymer sleeve (“CPPS”), involves providing a seamless cylindrical tube of photopolymer (a “CPPS plate”). The tubular CPPS plate may be mounted on the drum of a plate preparation device and/or a printing press by sliding the tubular CPPS plate axially over the cylindrical surface of the drum. The inner diameter of the tubular CPPS plate is typically sized for a snug fit on the cylindrical surface of the drum.
CPPS plates suffer from a number of limitations. It is difficult to fabricate seamless CPPS plates, making them relatively expensive in comparison to conventional flat printing plates. CPPS plates are inefficient to transport, because the tubular-shaped CPPS plates occupy a large volume for the amount of printable surface area which they provide. Handling of CPPS plates is difficult, particularly after the sensitive LAMS layer (or other surface mask layer) has been applied. The use of CPPS plates may also be inefficient where an image to be printed occupies only a small region of the CPPS plate.
In an alternative technique known as “plate-on-sleeve”, one or more sections of initially flat printing plate are cut to size and then applied to (i.e. wrapped around) the cylindrical surface of a tubular sleeve. Such tubular sleeves may be made from a variety of materials known in the art. Typically, the one or more sections of printing plate are mounted on the cylindrical surface of the tubular sleeve using a specialized plate-on-sleeve plate mounting apparatus. The one or more plate section(s) may be fastened to the tubular sleeve by any of a wide variety of techniques known in the art. The tubular sleeve bearing the one or more sections of printing plate may be mounted on the cylindrical drum of a plate preparation device and/or a printing press by sliding the tubular sleeve axially over the cylindrical surface of the drum. The inner diameter of the tubular sleeve may be sized for snug fit on the cylindrical surface of the drum.
Plate-on-sleeve preparation of printing plates has a number of advantages over CPPS techniques. Plate-on-sleeve printing plates may be fabricated and transported as flat sheets which may be subsequently cut or otherwise divided into appropriately sized plate section(s) and applied to the tubular sleeve, reducing fabrication and transportation costs. Once the plate section(s) are mounted on the tubular sleeve, non-plate bearing regions of the sleeve may be used for handling. In addition, the printing plate section(s) may be sized to fit the images being printed, maximizing the efficiency of plate usage.
One difficulty common to all techniques for preparing flexographic printing plates having LAMS layers or other types of integral surface masks occurs at the edges of the plate. FIG. 1-A shows a printing plate 10 having a base layer 11, a photopolymer layer 12 and an integral surface mask layer 14. Surface mask layer 14 may be a LAMS layer, for example. In the illustration of FIG. 1-A, surface mask layer 14 has already been imagewise patterned (for example, by selective ablation) to create a mask having UV-transparent regions 16 and UV-opaque regions 18. UV radiation source 20 produces a flood illumination field, schematically depicted by arrows 22. Actinic radiation field 22 exposes the areas of photopolymer layer 12 under UV-transparent mask regions 16, while the areas of photopolymer layer 12 located under UV-opaque mask regions 18 are not exposed. The exposure to actinic radiation changes the properties of photopolymer layer 12 in the exposed regions.
FIG. 1-A shows that surface mask layer 14 (which has been sprayed or roll coated onto the top of photopolymer layer 12) does not cover the peripheral edges 27 of plate 10. While arrows 22, which schematically depict the flood actinic radiation field, are shown as perpendicular to the surface of printing plate 10, those skilled in the art will appreciate that the flood radiation also spreads laterally from UV source 20, as shown by arrows 24. As a result of this spreading, actinic radiation 24 incident on unmasked edges 27 of plate 10 at least partially exposes edge regions 26 of photopolymer layer 12. The exposure of edge regions 26 at least partially changes the properties of the photopolymer therein.
After UV exposure, printing plate 10 is processed, to produce the plate 10 shown in FIG. 1-B. Processing typically involves the use of solvents and/or other procedures to remove LAMS layer 14. In the illustrated embodiment, processing also involves removing the unexposed regions of photopolymer layer 12 to form recessed regions 28. As shown in FIG. 1-B, recessed regions 28 are formed where photopolymer layer 12 was masked by the UV-opaque mask regions 18 of LAMS layer 14. In practice, there may be a base level 30 of photopolymer layer 12 which is formed, for example, by prior UV exposure (not shown) originating from the underside of plate 10. Processing does not remove photopolymer from regions 29, where photopolymer layer 12 has been exposed. As a result, regions 29 provide relief with respect to base level 30 in regions 28. When subsequently used in a printing press (not shown), ink is applied to the upper surface of plate 10. Because of their relative relief, ink applied to regions 29 is transferred to the material being printed, whereas ink applied to recessed regions 28 is not transferred.
The exposure of edge regions 26 to actinic radiation 24 results in the formation of ridges 32 (FIG. 1-B). Ridges 32 cause problems because they are not part of the desired image and when ultimately used in a printing press, ink applied to ridges 32 may be undesirably transferred to the material being printed.
In conventional (i.e. flat) preparation of flexographic plates, ridges 32 may be trimmed before the plate is mounted on the cylindrical drum of a printing press. However, ridges 32 present a particular difficulty when printing plates are prepared “in the round”, because it is difficult to trim ridges 32 from CPPS plates or from plate section(s) which have already been affixed to a sleeve in a plate-on-sleeve process.
U.S. Pat. No. 6,326,124 (Alince et al.) describes a UV-opaque edge covering material which may be manually brushed or sprayed onto the edges of a printing plate prior to UV exposure to reduce the occurrence of ridges. Other known techniques involve the manual application of UV-opaque tapes or strips to the edges of plates prior to exposure.
There is a general need to reduce or eliminate ridges and/or similar effects which occur at the edges of flexographic printing plates during plate preparation. There is a particular need for automated apparatus and methods for accomplishing this objective.